The city is not too big and public transport can be blackrided.
Hospitality exchange community is not as big as Sofia, you might have difficulties finding a host, especially in springtime-summer while lots of travelers and tourists pass by here..
Places There is a pedestrian street in the center of town called Ulitsa Knyaz Alexander which has many tourists and shoppers during the day. The best time to busk is probably the mid afternoon, as there is not too much nightlife in this town. There are ruins and a square on this street where people tend to gather.
Also, throughout much of the town there are underpasses which go under the roadway. During the day they have a nice amount of traffic and could offer good acoustics for potential street performers.
Another possible location is on the hill in the old town.
Law- Police From 2013 on, you DO NEED A PERMIT to busk in this street, police will come almost straightforward to ask for it. It is not very expensive though it takes 2 weeks to be issued at the town hall and if you're just passing by might be hard find time and papers for it. The fine for busking with no permit is 30lev (15€) and shouldn't be confiscation of your instrument, though might depend on the police officer.. In any other place they won't ask you anything..except in this main pedestrian street and they can be quite mean..as always as a foreigner you can escape the fine, telling you didn't know, you don't understand, won't do it anymore etc..though be aware of this..